Valve actuation in an internal combustion engine is required in order for the engine to produce positive power and may also be used to provide engine braking. Typically, engine valves may be actuated in response to the rotation of cams. One or more lobes on the cam may displace the engine valve directly, or act on one or more valve train elements, such as a push tube, rocker arm, or other mechanical element connecting the cam to the engine valve. During positive power, intake valves may be opened to admit air and sometimes fuel into a cylinder for combustion. Intake valves may also be opened to permit exhaust gas recirculation (EGR) back into the intake manifold. The exhaust valves may be opened to allow combustion gas to escape from the cylinder during main exhaust or an engine braking event, as well as for EGR.
During engine braking, the exhaust valves may be selectively opened to convert, at least temporarily, an internal combustion engine of compression-ignition type into an air compressor. This air compressor effect may be accomplished by cracking open one or more exhaust valves near piston top dead center position for compression-release type braking, or by maintaining one or more exhaust valves in a cracked open position for much or all of the piston motion for bleeder type braking. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle. A properly designed and adjusted engine brake can develop retarding horsepower that is a substantial portion of the operating horsepower developed by the engine during positive power.
For both positive power and engine braking applications, the engine cylinder intake and exhaust valves may be opened and closed by fixed profile cams in the engine, and more specifically by one or more fixed lobes, which may be an integral part of each of the cams. The use of fixed profile cams can make it more difficult to adjust the timings and/or amounts of engine valve lift needed to optimize valve openings and lift for various engine operating conditions, such as different engine speeds.
One method of adjusting valve timing and lift, given a fixed cam profile, has been to incorporate a “lost motion” device in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic or other linkage means. Some lost motion systems may be adapted to selectively vary the amount of lost motion on an engine cycle-by-cycle basis and/or to provide more than two levels of valve actuation during engine operation and are referred to as Variable Valve Actuation (VVA) systems.
Some lost motion hydraulic valve actuation systems, whether they are VVA systems or not, may include two hydraulically linked pistons; a master piston and a slave piston. Master and slave pistons may be elongated cylindrical structures that are adapted to slide in and out of bores in a common housing with a hydraulic passage connecting the two bores. A motion used to actuate an engine valve, such as a cam motion, is input to the master piston. The displacement of the master piston by the cam lobe is transferred to the slave piston via the hydraulic linkage connecting the two. When a sufficient amount of the master piston motion is transferred to the slave piston, the engine valve(s) connected to the slave piston may be actuated.
A solenoid valve may be connected to the hydraulic linkage between the master piston and the slave piston. The solenoid valve may be selectively opened to release fluid from the hydraulic linkage, which may prevent the master piston motion from being transferred to the slave piston. One primary distinction between WA and non-VVA lost motion systems may be the speed at which the solenoid valve is capable of release fluid from and refilling the hydraulic linkage between the master and slave pistons. VVA systems often have “high-speed” trigger valves serving in this capacity in order to adjust valve timing on an engine cycle-by-cycle basis.
In some lost motion hydraulic valve actuation systems, the slave piston may be used to open more than one engine valve at a time. For example, many engines employ two or more exhaust valves and two or more intake valves per cylinder. A single slave piston may be used to actuate multiple exhaust or multiple intake valves by acting through a valve bridge. The force required to open engine valves can be substantial, particularly when exhaust valves are opened for compression-release type engine braking. The pressure in the hydraulic linkage between the master and slave pistons that is required to open the engine valves is related to the diameter of the slave piston. The greater the diameter of the slave piston, the lower the hydraulic pressure in hydraulic linkage required to exert a given valve actuation force. Elevated pressures in the hydraulic linkage between the master and slave pistons may mandate thicker and heavier housing walls, place higher stresses on the valve actuation system components, produce greater pressure oscillations in the linkage, and/or may make the system more susceptible to leakage and failures.
Accordingly, there is a need for a hydraulic valve actuation system that can produce lower and/or more stable pressures in the system hydraulic circuit. Theoretically, lower and more stable pressures in the hydraulic circuit could be achieved by increasing the slave piston diameter. There is a limit, however, on the size of the slave piston that may be used in a hydraulic valve actuation system. This limit is imposed by the space constraints of modern engines. Accordingly, there is a need for a hydraulic valve actuation system that produces lower and/or more stable pressures in the system hydraulic circuit while at the same time meeting component size limitations for the engine.
As noted above, many engines employ multiple intake and exhaust valves per engine cylinder. Known hydraulic valve actuation systems utilizing a single slave piston have required the use of a valve bridge to transfer valve actuation motion to multiple engine valves. The need to include a valve bridge may add to the complexity, cost, and space requirements of the valve actuation system. Accordingly, there is a need for a valve actuation system in which slave piston actuation may be transmitted to more than engine valve without the need for a valve bridge.